Field of the Invention
The invention relates to a method for identifying erratic combustion in multicylinder internal combustion engines, i.e. misfire in cylinders, wherein the angular speed of the crankshaft is measured and which permits dependable recognition of misfirings, even in case of errors in the measurement of the crank angle speed.
In engines equipped with a catalytic converter, misfiring can lead to damage to the converter because high temperatures occur in the catalytic converter due to the secondary reaction of the unburned fuel-air mixture. The quality of waste gas is worsened by misfiring, regardless of whether or not a catalytic converter is used.
Various methods are known in which the instantaneous crank angle speed of the crankshaft is measured for identifying misfiring. A misfiring cylinder causes a temporary slowing of the angular speed of the crankshaft. To identify this, the periods of time (segment times) are measured during which the crankshaft rotates about a defined angular span. Such time measurement is conventionally done with markings on the crankshaft or a shaft connected to the crankshaft, or with a wheel mounted on these shafts. The markings or the wheel are scanned with sensors suited for this purpose.
The measured segment times are not used directly. Rather, so-called fluctuation values in the rotational speed are calculated from them, and various dynamic influences occurring during operation of the engine (acceleration, deceleration) can be compensated, depending on the method employed. A method of this type is described in European Patent Application 92111078.9, for example.
The fluctuation values are then compared with predetermined threshold values and, when the fluctuation values are below these threshold values (provided that a method is used in which a negative fluctuation value results from a misfire), or when the fluctuation valises exceed these threshold values (provided that a method is used in which a positive fluctuation value results from a misfire), misfiring is identified. If need be, control provisions can then be taken, such as turning off the applicable injection nozzle.
These methods can only function properly without errors when no errors occur during a measurement of the segment times.
These errors can stem from varying segment sizes, inaccuracies in tooth shape, disk backlash and errors in the sensors, for example.
If a marked segment is larger than the adjacent segment, for example, a slowing of the crankshaft is mimicked and misfiring is falsely identified.